Amnesty International has condemned the Western Australian state government for missing its deadline to return children incarcerated at an adult prison to juvenile detention facilities in the state.

More than 100 children, the majority of them Indigenous Australians, have been housed at Hakea prison in Canning Vale, WA since January, following a riot at Banksia Hill juvenile detention facility – also in Canning Vale and the state's only juvenile detention institution – during which 108 of 240 cells were damaged. At a supreme court hearing in May, WA chief justice Wayne Martin described the conditions in which young people were held at Hakea as "less than optimal", but said the decision to transfer them to the facility following the January incident was legitimate.

During the hearing, the WA Department of Corrective Services said the children would be returned to Banksia Hill by the end of June, but the state government announced last week it had pushed back the deadline to the end of August in an apparent bid to ensure youth and staff safety at Banksia.

"We are very concerned at reports the children will now remain in Hakea until at least August this year, meaning some of the children in the adult prison will have been there for close to a year," said Rodney Dillon, Amnesty International Indigenous rights campaigner.

"This is the third date the department has committed to returning the children to appropriate facilities. It has once again passed; despite the department's own commitment to heed the calls of countless organisations," he said.

Dillon said that Amnesty had heard reports that some of the children taken back and forth between facilities at Hakea and Banksia Hill had been subjected to "repeated strip searches" because of movement between the two facilities.

"The department must prioritise the children's needs, and move them to a refurbished Banksia Hill juvenile detention centre urgently, addressing the staffing shortages that have seen kids locked in their cells for as long as 23 hours at a time," Dillon added.

The May court case revealed the shocking state of youth detention provision in the state, which has the highest rate of youth Indigenous detention in Australia. The court heard how the ratio of detainees to adults in WA was one to eight, versus a national average of one to four. It also heard how, before the January incident, detainees at Banksia were subject to rolling lockdowns due to crippling staff shortages.

WA's Corrective Services department says it has spent $1.5m repairing the Banksia Hill centre, but its youth custodial officers have said they refused to return until safety concerns were addressed.